Tech Series Soils

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Soils in Construction

Soils and Sitework

Earthwork: support intended use cut and fill material nature and location of rock

Footings and bearing capacity Foundations: structural design

Drainage surface drainageRequirements: subsurface drainage

susceptibility to erosiondry wellsseptic systems

Fine Homebuilding

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Soil Exploration

Land Use Feasibility: geological maps soil surveysUSGS maps on-site analysis

Construction Documents: site-specific, detailed informationsoil borings and test pits

Critical Conditions: organic soils, silt clays, loose siltsfine water-bearing sandhigh water tablerock close to surfaceland fills, dumps, unconsolidated fillsevidence of earth movement

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Soil Investigation

Fine Homebuilding

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Soil Investigation

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Soil Characteristics

Phase Diagrams

Weight

Wt = Ww + Ws

Volume

Vt = Vv + Vs= Va + Vw + Vs

From SOIL MECHANICS, College of Engineering, Florida State University

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Soil Density

May be referred to as: Bulk Density Unit Weight

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Water Content

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Void Ratio

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Porosity

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Soil Classification Systems

United States Department of Agriculture (USDA) Soil Triangle

American Association of State Highway and Transportation Officials(AASHTO)

Federal Aviation Administration (FAA)

Standard Soil Classification System (ASTM D2487)Formerly the Unified Classification System, the most common system used in construction.

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Standard Soil Classification System ASTM D2487

Based primarily on those characteristics that indicate how soil willbehave as a construction material.

Classification procedure includes:

Sieve (mechanical) Analysis (ASTM D422) Particle Size Distribution Curve Liquid and Plastic Limit

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Sieve Analysis

Sieve Analysis

Soil is passed through a set of sieves with openings of known sizes and then the amount of soil retained on each sieve is weighed.

Sieve analysis is used to classify coarse soils sands and gravels.

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Sieve Analysis

Portland Cement Association

Soil Fraction Size Range

Cobble > 3

GravelCoarse 3 to Fine to No. 4 Sieve

SandCoarse No. 4 to No. 10Medium No. 10 to No. 40Fine No. 40 to No. 200

Fines (silt or clay) < No. 200

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Unified Soil Classification System: Coarse Grained

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Understanding Sieve Analysis Curves

GRAVEL

Above No. 4 Sieve

SAND

Between No. 4 Sieve And No. 200 Sieve

SILT/CLAY

Below No. 200 Sieve

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Sieve Analysis Curves: Well Graded Gravel

Curve 1

Coarse Fraction: 97%Retained Above Sieve No. 4: 75%

Curve 2


This is not more than 50% of the coarse Fraction, therefore SOIL 2 is a SAND.

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Definitions

Well-graded: a coarse grained soil (sand or gravel) with a widerange of particle sizes. Also referred to as:

Dense graded

Poorly-graded: a coarse grained soil (sand or gravel) with a limitedrange of particle sizes. Also referred to as:

Open gradedUniform gradedGap graded

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Unified Soil Classification System: Coarse Grained

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Sieve Analysis Curves: Well Graded Sand

Curve 1


Curve 2


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Particle Size Distribution

Uniformity Coefficient Cu (measure of the particle size range)

Cu = D60/D10Cu < 5: Very Uniform Cu = 5: Medium Uniform Cu > 5: Non-uniform

Coefficient of Gradation or Curvature Cg [Cc](measure of the shape of the particle size curve)

Cg = (D30)2/ D60 x D10

Cg from 1 to 3: well graded

Where: D10 = diameter of particle size at 10% passingD30 = diameter of particle size at 30% passingD60 = diameter of particle size at 60% passing

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Sieve Analysis Curves: Well Graded Sand

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Definitions

Liquid Limit: the water content, as determined by the standard (LL) liquid limit test (ASTM D423), at which a soil passes

from a plastic to a liquid state.

Plastic Limit: the lowest water content, as determined by the (PL) standard plastic limit test (ASTM D424), at which a

soil remains plastic.

Plasticity Index: the difference between the liquid limit (LL) and plastic(PI) limit (PL), or the range of water contents over which a

soil exhibits plastic behavior.

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Behavior Analysis

Solid Semi-Solid Plastic Liquid

SL PL LL

Increasing water content

Plasticity Index (PI) = LL PL

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Sieve Analysis Curves

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Unified Soil Classification System: Fine Grained

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Soil Classification


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Frost Penetration

Ice layers grow from top down.

Conditions necessary to increase ice thickness:

Freezing temperature in soil Water table close to frost table Characteristics necessary for rapid capillary action

Soil heaves equal to ice thickness causing pavementdisplacement

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Frost Penetration

Methods to reduce or prevent frost action:

Use non-frost-susceptible base or sub-base Extend structure below frost line Use cut-off blanket between subgrade and sub-base/base Lower water table

Thawing creates a super-saturated soil, significantly reducing bearing capacity.This condition often results in pavement failure.

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Shear Strength

The stability of a soil and its ability to resist failure under loading is derived from itsshear strength. The shearing strength of a soil is the result of friction between particles and cohesion. Shear strength is not constant, since cohesion varies with water content, rate and duration of loading, confining pressures and other factors.

Granular Soils have internal friction resistance. have no cohesion. strength increases with increased normal pressure. safe slopes of granular bank do not decrease with increased height.

Cohesive Soils have cohesive bonding. have no internal friction. strength is the same regardless of normal pressure. safe slopes of cohesive bank becomes flatter as bank height increases, sincestrength does not increase.

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Shear Strength

granular soil

cohesive soil

heights

t

r

e

n

g

t

h

In reality, most soils consist of bothgranular and cohesive components, thus deriving strength from internalfriction and cohesion.

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Swell and Shrinkage

Swelling and shrinkage are the result of a build-up and release of water within the soil pore spaces.

Fine grained soils most susceptible to swelling and shrinking.

Shrinkage in clay very slow, while rapid in granular soils.

Shrinkage affects settlement on clay more than on granular soils.

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Soil Compaction


The degree of compaction of soil is measured by its unit

weight (or density) and optimum moisture content.

The process of soil compaction is simply

expelling the air from the voids or reducing air voids.

As soil is compacted, soil density is increased.

Reducing, or squeezing, water from the voids is

referred to as consolidation, not compaction.

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Soil Compaction

In the construction of streets, parking areas, embankments and many other site development projects, it may be necessary to compact soils to increase their density.

Compaction improves the following characteristics of soils for engineering purposes: Increases Strength Decreases permeability Reduces settlement of foundation Increases slope stability of embankments

Soil compaction can be achieved either by static or dynamic loading: Smooth-wheel rollers Sheepsfoot rollers Rubber-tired rollers Vibratory Rollers Vibratory Tampers

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Placing and Compacting Soil

Placement of Fill Material

Topsoil should be removed in fill areas

If area is to support structures (pavements, footings, foundations, etc.), subgrade should be compacted according to engineers specifications, including depth, density and moisture content.

To minimize settlement, fill should be placed and compacted in layers, referred to as lifts.

Fine Homebuilding

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Fine Homebuilding

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Compaction

Compaction specifications for structural soils should include density and optimum moisture content.

Range of optimum moisture content: Sand 8% Silt 15% Clay 15 20%

Water content should be controlled in making fills Control stormwater runoff. Discharge water away from trenches and excavations. Trenches and excavations should be dewatered.

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Compaction Methods

Rolling is the primary method. Three basic roller types:

Steel-wheeled: recommended for hard, angularmaterial. Rubber tired: recommended for softer material(sandy soils) and for rolling during final shapingoperations. Sheepsfoot or tamping: recommended for clay.

Heavy rubber-tired earth moving equipment such asdump trucks assist in compaction during deposition ofsuccessive layer.

Rammers, tampers and plate vibrators are used to compacttrench excavations, excavations in tight spaces, and againstretaining and foundation walls.

Fine Homebuilding

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Controlled Fill

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Controlled Fill

grade stakes

organic and clay soils

controlled sand fill

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Subgrade for Pavement

Removing poor subgrade soils after pavement failure.

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Removal of topsoil and preparation of subgrade for asphalt pavement.

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Final preparation of subgrade. Base and sub-base material to be placed on top of prepared subgrade. Base is opengraded gravel, while sub-base is densegraded gravel.

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Geotextiles

Geotextiles apply to a broad range of civil engineering construction, paving, drainage, and other applications.

Engineered geotextiles perform three basic functions: separation stabilization filtration

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Geotextiles

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Geotextiles

Woven Geotextiles are generally preferred for applications where high strength properties are needed, but where filtration requirements are less critical and planar flow is not a consideration.

Nonwoven Geotextiles are needle-punched continuous filament engineering fabrics capable of providing planar water flow in addition to their soil stabilization and separation functions. Typical applications include access roads, aggregate drains, asphalt pavement overlays, and erosion control.

For applications such as roadways, parking lots, loading areas, and construction sites, geotextiles reduce subsoil migration into the aggregate base course, enhancing the long-term performance of the base course while preserving its drainage and structural capacities.Under heavy traffic and construction loads, Woven Geotextiles reduce localized shear failure in weak subsoil conditions.

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Geotextiles

Confinement: Maintains base thickness and high friction, thus maintaining design strength of base course.

Load Distribution: Reduces localized stress by distributing loads over wider area of subgrade.

Separation: Serves as a barrier between fine grain soils and aggregates. Eliminates the loss of aggregates into subgrade and the pumping of silt and fine grain soils into aggregate.

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Specifications

Construction Specifications Institute MasterSpecRelevant Soil Specification Sections:

2210 Subsurface Investigation2230 Site Clearing (stripping & stockpiling topsoil)2310 Grading2315 Excavation and Fill2340 Soil Stabilization2370 Erosion and Sedimentation Control